Heat guns, pistol-shaped devices emitting heated air from a "barrel," are in wide use by homeowners, contractors and the like for defrosting, forming plastics, stripping paint and many other uses. Such heat guns also resemble (in outward appearance and general function) common household hair dryers. However, hair dryers are relatively simple and devoid of temperature regulators.
Such heat guns are used for a variety of purposes, each of which may require (or at least prefer) a different air temperature. For example, paint stripping may require a very high air temperature while plastic forming, especially forming thin sheets or "small-mass" pieces will likely require a more moderate temperature. And, sometimes, the gun is employed solely for its forced air stream with no need for the air to be heated.
Certain known temperature regulators use low-power circuitry having solid state amplifiers. In prior art heat guns, the voltage change of the temperature-sensing thermocouple is amplified using an amplifier with a variable gain. The amplified thermocouple voltage is applied to a comparator which compares such voltage with a fixed threshold or reference voltage and switches a thyristor accordingly.
U.S. Pat. No. 3,443,121 (Weisbrod) depicts a temperature control system uses a variable resistor to adjust gain. And the sawtooth generator circuit also includes an adjustable resistor. Therefore, production versions of the Weisbrod circuit would apparently require adjustable potentiometers, the ohmic values of which are closely matched. There is no mention of protection against an open thermocouple.
The system shown in U.S. Pat. No. 3,684,172 (Evalds) uses a variable resistor to select a "set point" temperature by adjusting gain on an amplifier. The system depicted in the Evalds patent also has a temperature compensating circuit which uses a thermistor as part of a circuit to "neutralize" the effect of cold junction voltages. Protection against an open thermocouple is not indicated.
Known temperature regulators of the foregoing types share certain disadvantages. A major disadvantage of those using rheostatically-adjusted, variable gain amplifiers (as depicted in the Weisbrod and Evalds patents) is that the ohmic values of the rheostats or potentiometers used in production heat guns must have closely similar values, e.g., within plus or minus 5% or so.
This is so since any deviations from the resistance "design value" will be amplified and there will be relatively large temperature "offsets." To put it another way, a particular potentiometer setting for each of a group of production heat guns will result in large variations in output air temperatures for such guns. The products will not perform uniformly one to the other. Potentiometers with the required parameters are readily available, but they are usually of large size and of high cost.
Another disadvantage of certain types of temperature regulators used in heat guns and other applications involves a type of control aptly known as "on-off" control. Some background information will be helpful in understanding some of the problems associated with on-off control.
Certain types of control systems are known as "closed loop" systems. In such systems, some "action" occurs, e.g., application of electrical power, and this causes a reaction, e.g., increase in the temperature of the air from a heat gun. The reaction (the rise in air temperature) is sensed and causes an adjustment to the action, e.g., switching the power off. Each such system also has a certain "time constant." This means that the action and the reaction do not occur simultaneously--there is a time lapse between them.
In a heat gun, the time constant is the time required to detect a change in the input power by a change in the output air temperature. For example, when electrical power is applied to the resistance heater of a heat gun, the heater wire gets hot and heats the ceramic core upon which it is wound. Of course, the air passing through the core is also heated. To put it another way, the time constant is the time required to increase the temperature of the heater wire, the ceramic core, the air stream being heated by the core and, finally, a thermocouple-type temperature sensor, the latter by an amount detectable by the regulator circuit.
The temperature sensor produces a small voltage, the value of which is a function of the sensed temperature. Such voltage is sometimes referred to as the Seebeck voltage after its discoverer. The sensor is heated by the air and in a conventional heat gun, the small sensor output voltage is amplified and applied to a comparator which, in turn, causes a switch to turn the power on or off.
Typically, the power is turned off only when the desired air temperature (often referred to as the "set point" temperature) is attained. But even after the power is turned off, a considerable amount of energy in the form of heat is stored in the resistance wire and in the ceramic core. This energy will continue to heat the air to a temperature higher than the set point temperature.
Conversely, as the air cools and its temperature declines below the set point temperature, electric power is again applied to the resistance heater. However, it will take some time to bring both the resistance wire and the ceramic core to a temperature sufficiently high to raise the air temperature back up to the set point temperature. Stated another way, with on-off control, there are relatively large temperature excursions above and below the set point. Such a system is said to exhibit poor regulation.
To combat these undesirable temperature excursions, a technique called proportional voltage control is often employed in more advanced temperature regulators. In proportional voltage control, the amplified sensor voltage is modulated by a second voltage which rises and falls over a period of time. Such second voltage displays essentially the shape of a sawtooth, the amplitude and period of which are designed to complement the controlled systems time constant. Conventional proportional voltage control systems employ comparators which compare the modulated sensor voltage with a threshold or reference voltage. The "compared" difference between such voltages affects the length of time that electrical power is applied to the heating element.
When this modulated sensor voltage is compared to the threshold voltage, the electric current will be applied to the resistance heating element for shorter or longer periods of time, depending, respectively, on whether the sensed (i.e., actual) air temperature is close to or more disparate from the set point temperature. Any significant overshoot of the set point temperature is usually avoided.
A drawback of conventional proportional temperature controls is that the amplitude (value) and period (time required for one cycle) of the modulating sawtooth voltage can be optimally matched to the system time constant only for a relatively small temperature span. Therefore, it is not ideally suited for the output of a heat gun requiring a temperature span from ambient to more than 1000.degree. F.
Another concern in the design of a temperature regulator is the possibility of a sensor failure. In some designs, such a failure functions to turn the electrical power applied to the heating element "full on." The heating element is therefore likely to overheat since that component which "tells" the regulator that a desired air temperature has been reached, the sensor, is inoperative. Although protection against a failed sensor is readily achieved, conventional regulators use several parts to accomplish this result. The cost of and space required by the regulator are therefore unnecessarily increased.
Yet another problem in conventional temperature regulators involves the "cold" connections of conductors of dissimilar metals in the regulator circuit. Such connections exhibit what is called a "cold junction" voltage. These cold junction voltages diminish or subtract from the thermocouple hot sensor voltage, thereby reducing the already-small value of the latter, As a consequence, accurate temperature control becomes more difficult since the regulator must "work with" a signal voltage of unnecessarily-reduced value.